1. Field of the Invention
The present invention relates to a crucible holding member and a method for producing the crucible holding member.
2. Description of the Related Art
A carbon material has heretofore been heavily used in a silicon single crystal pulling-up apparatus, for the reasons that the carbon material has high heat resistance and high thermal shock properties, and that the carbon material hardly contaminates silicon. In particular, an isotropic graphite material is hard to react with a reactive gas such as SiO generated in the apparatus due to its high density, and the reaction rate of the isotropic graphite material with SiO2 as a material for a quartz crucible for containing a silicon melt is small. Accordingly, the graphite member has been widely used as a crucible holding member for holding the periphery of the quartz crucible.
In recent years, an increase in diameter of a silicon wafer has progressed in order to increase yield and improve productivity, and a 300-mm wafer has been becoming mainstream. The development of a wafer further increased in diameter exceeding 400 mm has also been advanced. With this increase in diameter of the silicon wafer, the seize of the silicon single crystal pulling-up apparatus becomes large, so that the weight of a crucible holding member used in the apparatus becomes extremely heavy, resulting in the difficulty of handling such as setting to the apparatus.
Further, a production process of the isotropic graphite material requires a press process under hydrostatic pressure, and requires a Cold Isostatic Press (CIP) apparatus having a size of about 1.5 times the diameter of the graphite product. The diameter of the conventional CIP apparatus is not enough for the isotropic graphite material as a large-size crucible holding member, so that a larger apparatus becomes necessary.
As a technique for producing the large-size graphite crucible holding member without using the CIP apparatus, there has been proposed a technique including forming carbon fibers into a specified form by a filament winding process, impregnating it with a resin or pitch as a matrix, and burning it to produce a crucible holding member made of a carbon/carbon fiber composite (hereinafter referred to as a C/C composite) (for example, see JP-A-10-152391 or JP-A-11-60373), and a technique including adhering carbon fiber cloth to a forming die, performing molding and curing to obtain a carbon fiber-reinforced plastic, and then, impregnating and burning it to produce a crucible holding member made of a C/C composite (for example, see JP-A-10-245275), or the like.
In the meantime, in the silicon single crystal pulling-up apparatus, a single crystal ingot is produced while melting silicon, so that it is necessary to heat the inside of the apparatus to a temperature equal to or higher than the melting point (1,420° C.) of silicon. When silicon is melted, the crucible holding member and the quartz crucible inserted therein are softened to adhere to each other. The coefficient of thermal expansion of quartz glass is 0.6×10−6/° C., and that of the C/C composite is generally equivalent thereto. Accordingly, when the apparatus is cooled after the silicon melt has been almost removed after completion of pulling-up of the single crystal ingot, both are cooled without being strongly restricted with each other.
However, when the silicon melt coagulates by a trouble such as a power failure immediately after the pulling-up is initiated, silicon has the property of expanding (a volume expansion of about 9.6%) with coagulation. Accordingly, this acts as the function of enlarging the quartz crucible and the crucible holding member.
In the case of the apparatus for pulling up a small-diameter single crystal ingot, even when such a trouble occurs, cooling is performed for a short period of time, and moreover, the amount of the non-coagulated melt leaked out is small. However, in the case of the apparatus for pulling up a large-diameter single crystal ingot, when such a trouble occurs, it takes time for cooling, and once the melt start to be leaked out, a large amount of the melt flows out to a bottom portion of the apparatus, which causes significant damage.
The crucible holding member made of the C/C composite produced by using the filament winding process as described in the above-mentioned publication JP-A-10-152391 or JP-A-11-60373 has extremely high strength because of existence of a large number of carbon fibers wound in a direction parallel to a circumferential direction thereof, so that this crucible holding member is suitable for a large-size crucible holding member. However, when the above-described trouble occurs, the carbon fibers aligned in the circumferential direction of the crucible holder member are pulled by very large force caused by expansion of the silicon melt at the time of its coagulation. Accordingly, the carbon fibers would break to cause damage to the crucible holding member.
Further, also in the crucible holding member produced by adhering the carbon fiber cloth as described in the Publication JP-A-10-245275, a large number of carbon fibers aligned in the circumferential direction exist. Accordingly, the crucible holding member would be damaged by tension applied in the circumferential direction, similarly to the above.
Furthermore, in a production process of the crucible holding member made of the C/C composite described in the above-mentioned publications, the carbon fibers are wound on or the carbon fiber cloth is adhered to the forming die to form a shape, a matrix precursor such as a resin is impregnated in the carbon fibers or the carbon fiber cloth, and heat curing and burning carbonization are performed together with the forming die, followed by releasing from the forming die. In these steps, strong tension is also applied to the carbon fibers due to the difference in the thermal expansion coefficient between the forming die and the crucible holding member made of the C/C composite, which may cause the breakage of the carbon fibers.
In addition, although the C/C composite is a material having excellent strength, it has a structural limit in the requirement for obtaining higher strength because the related-art crucible holding member made of the C/C composite has a monolayer structure. The quartz crucible keeps on increasing the size thereof, and with the increase in size, it is expected that very large force comes to act on the crucible holding member. For this reason, further improvement in strength has been desired for the crucible holding member. Further, since the related-art crucible holding member made of the C/C composite has the monolayer structure, an end of the carbon fiber or carbon fiber cloth becomes a broken end at a periphery of an opening portion to cause fraying, resulting in a significant decrease in strength of the opening portion.
Such disadvantages are not limited to the crucible holding member for the silicon single crystal pulling-up apparatus, but similar problems occur in the above-mentioned various fields in which a member contains a container different therefrom in the thermal expansion coefficient in the inside thereof. It has therefore been desired to develop a crucible holding member having sufficient strength to support a container having heavy weight and suppressing occurrence of cracks and the like even when tension occurs in the circumferential direction.